In recent years, surface light-emitting EL lightings have been attracting attention as next-generation lightings. Surface light-emitting EL lightings have benefits such as high luminescence uniformity, thinness and space-saving, high designability, and eye-friendly. Although some surface light-emitting EL lightings including organic EL are already commercially available, there are still many challenges for widespread use thereof because of cost and durability. Moreover, in the case where a light emitting diode (LED) is used, the use of an expensive substrate is required, thus posing a problem in that providing large-area devices is difficult.
Meanwhile, zinc oxide (ZnO) is a safe, inexpensive compound semiconductor and is known as a material that is chemically stable and also has excellent transparency. Zinc oxide is mainly used in the form of a sintered body or a powder, and is utilized for sputtering targets, varistors, rubber additives, cosmetics, and the like. In the case where zinc oxide is used as a sintered body, it is known that aligning the crystal orientation of zinc oxide changes its characteristics. For example. Patent Document 1 discloses a crystallographically oriented zinc oxide sintered body with a (101) crystal orientation ratio falling within a predetermined range as a sputtering target. Patent Document 2 discloses a (110)-oriented zinc oxide sintered body.
Furthermore, an application of zinc oxide to light emitting diodes (LEDs) has been investigated, and, for example, Non-Patent Document 1 reports that ultraviolet emission was observed from an LED produced by forming a N-doped MgxZn1-xO film on a Zn-polar ZnO single crystal substrate.